The present invention relates to a method of drilling holes to the surface of a dryer cylinder in a paper machine, said dryer cylinder comprising a shell, a cavity enclosed by the shell and grooves made to the outer surface of the shell.
Today, it is conventional to provide the outer surfaces of dryer cylinders in a paper machine with grooves running circumferentially along the perimeter of the cylinder. These grooves in turn are provided with drilled holes serving to improve air flow through the web and simultaneously to aid the adherence of the web to the cylinder surface during the rotation of the cylinder. Such drilled holes are patterned so as to perforate the shell at the groove bottoms down into the hollow interior of the cylinder.
This arrangement, however, is hampered by the relatively small diameter of the holes drilled on the cylinder shell, typically about 4 mm. Due to the small size, the holes will gradually become plugged by the particulate matter separating from the web, whereby the function of the dryer cylinder is substantially degraded.
A remedy to the above-mentioned problem has been sought from a drilling pattern with a substantially high excess number of holes, whereby some plugging of the holes still leaves a major portion of the holes free to pass the air flow. Even this approach may involve the problem of regional plugging of all holes. Then, the web will be subjected to an uneven air flow causing bagginess in the web that is easily torn at the bags. On the other hand, the excess number of holes must be limited to avoid too large an air flow into the cylinder interior, thus causing an essential degradation of the dryer cylinder efficiency.
It is an object of the present invention to overcome the disadvantages of the above-described prior-art techniques and to provide an entirely novel arrangement permitting the shell of a dryer cylinder to be drilled in a reliable manner to assure a sufficient flow rate of air.
The goal of the invention is attained by virtue of the method specifications defined in appended claims. More precisely, the method according to the invention is principally characterized by what is stated in the characterizing part of claim 1.
In practice, the method according to the invention is implemented by restoring the air flow properties of a dryer cylinder in a paper machine through a redrilling operation adapted to open the holes on the cylinder shell.
The drilling technique according to the invention offers a cylinder reconditioning method that is capable of restoring the condition of the cylinder and even improving its function over the original performance, with a further benefit of being applicable onsite at the actual location of the cylinder. By virtue of enlarging the diameter of the original holes drilled on the cylinder shell through redrilling the holes into a slightly larger size, the rapid plugging of cylinder shell holes can be prevented without excessively increasing the air flow rate through the cylinder shell.
By drilling the holes from the cylinder interior outward, it is possible to avoid the formation of disturbing hole burrs that are conventionally raised during drilling on the outer surface of the cylinder shell.
It must be noted that the present method according to the invention can be used for both opening and widening the original cylinder shell holes that are plugged or expected to become plugged soon as well as for perforating the cylinder shell with entirely new holes.